The present invention relates to radio communication technology, and more particularly to apparatuses and techniques for compensating for DC-offsets and low frequency distortion introduced in a radio receiver.
Modem telecommunications systems, such as cellular telecommunications systems, rely on digital technology for the representation and transmission of information, such as audio information. Any of a number of modulation techniques are used to impose the digital information onto a radio frequency signal, which is then transmitted from a sender""s antenna, and which is received by a receiver""s antenna. Ideally, the receiver would merely perform a reverse of the modulation process to recover the digital information from the received signal.
In practice, however, the transmitted signal is often distorted by the channel (i.e., the air interface) between the transmitter""s antenna and the receiver""s antenna. For example, a main ray of a transmitted signal may take a direct route between the transmitting and receiving antennas, but other rays may take indirect routes, such as reflecting off of various objects (e.g, buildings, mountains) in the environment prior to being received by the receiver""s antenna. This effect is often called xe2x80x9cmultipath propagationxe2x80x9d of the signal. These indirect paths can take longer for the signal to traverse than the direct path. Consequently, signals representing the same information may arrive at the receiver at different times. The various paths between the transmitter and the receiver subject the signal to varying amounts of attenuation, so they are not all received at the same signal strength. Nonetheless, they are typically received at sufficiently high power levels to cause an effect wherein, at any moment, a received signal includes a present signal (representing a present piece of desired information) plus one or more delayed components from previously transmitted signals (each representing an earlier piece of information). This type of signal distortion is often called Inter-Symbol Interference (ISI).
To counteract ISI, a receiver typically employs an equalizer, which demodulates the signal in a way that utilizes a model of the channel (also referred to as an xe2x80x9cestimatexe2x80x9d of the channel). The channel estimate is typically generated from another component in the receiver, called a channel estimator. A channel estimator relies on a received signal including a portion, often called a xe2x80x9ctraining sequencexe2x80x9d, that contains a predefined sequence of 1""s and 0""s known to have been transmitted by the transmitter. By comparing an actually received training sequence portion of a signal with an expected training sequence, the channel estimator is able to construct a model of the channel that can be used by the equalizer when it attempts to demodulate a portion of the received signal that includes unknown information.
FIG. 1 is a block diagram of a conventional channel estimator and channel equalizer for use in a burst transmission system such as, for example, the Global System for Mobile communication (GSM) system. A received radio signal is down-converted to a baseband signal 101 by circuitry in a radio receiver (not shown). The baseband signal 101, which has a predefined burst length, is supplied to a memory 103, where it is stored. A synchronization unit 105 identifies that portion of the stored received signal that corresponds to the training sequence, and supplies these samples to a channel estimator 107. The channel estimator 107 computes the K:th order channel filter taps {hi}, (i=0, K) from the received signal while referring to a reference training sequence signal 102. The number of channel taps, K, is application specific, and will generally be specified based on the maximum expected delay spread for the received radio signal. The channel filter taps are then supplied to an equalizer 109, which may for example, be a Viterbi equalizer having MK states, where M is the number of possible symbols. The equalizer 109 then uses the channel estimate to demodulate those portions of the received baseband signal (as supplied by the memory 103) that correspond to unknown information. The output of the equalizer 109 are the decided symbols 111.
The conventional channel estimator 107 and equalizer 109 as described above assume that the baseband signal 101 does not contain a DC-offset or low frequency distortion. xe2x80x9cLow frequencyxe2x80x9d in this context is defined as a signal whose rate of change is slow compared to the dynamics of the radio channel and the rate of the transmitted information (e.g., the low frequency distortion is relatively constant over the span of two transmitted symbols). However, many types of radio receivers, especially homodyne receivers, introduce DC-offsets. These arise from, for example, component mismatch in the receiver in-phase (I) and quadrature phase (Q) paths; the signal from the local oscillator leaking into the antenna and becoming downconverted to DC or slowly varying DC in the mixers; or a large near-channel interfering signal leaking into the local oscillator and self-downconverting to a varying DC offset. The presence of DC-offsets and low frequency distortion in the signal supplied to the conventional equalizer 109 results in degraded performance, as measured by the accuracy with which the received signal is decoded.
The German patent document DE 196 06 102 A1, published on Aug. 21, 1997, suggests a method for compensating for a DC-offset in an equalizer by introducing a DC-tap in the channel model. However, this method cannot take care of low frequency distortion because it assumes a pure DC component, that is, the DC-tap is constant during the entire burst.
Thus, there is a need for a channel estimator and equalizer that explicitly take into account both the DC-offset and the low frequency distortion introduced in the radio receiver.
The foregoing and other objects are achieved in methods and apparatuses for equalizing a radio signal that has been received from a channel. In accordance with one aspect of the invention, equalizing the radio signal comprises estimating the channel; generating an initial estimate of a DC-offset that has been introduced into the radio signal; estimating a variance of the initial estimate of the DC-offset; and using the channel estimate, the initial estimate of the DC-offset and the estimated variance to determine a most likely symbol represented by the radio signal.
In another aspect of the invention, the operation of generating the initial estimate of the DC-offset that has been introduced into the radio signal comprises using a training sequence portion of the received signal and a reference training sequence signal to generate the initial estimate of a DC-offset that has been introduced into the radio signal.
In still another aspect of the invention, the operations of estimating the channel, generating the initial estimate of the DC-offset and estimating the variance of the initial estimate of the DC-offset are performed by using a Least Squares technique to determine the channel estimate, the initial estimate of the DC-offset and the variance of the initial estimate of the DC-offset from a training sequence portion of the received signal and a reference training sequence signal.
In yet another aspect of the invention, the operation of using the channel estimate, the initial estimate of the DC-offset and the estimated variance to determine the most likely symbol represented by the radio signal comprises determining a plurality of possible DC-offset values based on a model of DC-offset variation; for each possible transition from a state i to a state j in a decoding trellis, determining a plurality of first metric values in correspondence with the plurality of possible DC-offset values; determining which one of the plurality of possible DC-offset values corresponds to a best one of the plurality of first metric values, and selecting said one of the plurality of possible DC-offset values as an optimal DC-offset value; using the optimal DC-offset value to determine an accumulated metric associated with the transition from state i to state j; finding a lowest accumulated metric associated with a best path through the decoding trellis to state j; and determining the most likely symbol represented by the radio signal based on the lowest accumulated metric associated with the best path through the decoding trellis to state j.
In still another aspect of the invention, the operation of using the channel estimate, the initial estimate of the DC-offset and the estimated variance to determine the most likely symbol represented by the radio signal comprises determining a plurality of possible DC-offset values based on a model of DC-offset variation; and using a delayed decision feedback sequence estimation technique to determine the most likely symbol represented by the radio signal, wherein the delayed decision feedback sequence estimation technique determines a metric that is based, in part, on the plurality of possible DC-offset values.